1. Field of the Invention
The present invention relates to viscous fluids for treating subterranean zones penetrated by well bores which include delayed viscosity breakers.
2. Description of the Prior Art
Viscous fluids are commonly utilized for treating subterranean zones penetrated by well bores. One such treatment is gravel packing. In gravel packing operations, solid gravel particles such as sand are carried into a subterranean zone in which a gravel pack is to be placed by a viscous fluid. That is, the gravel is suspended in the viscous fluid at the surface and carried to the subterranean zone in which a gravel pack is to be placed. Once the gravel is placed in the zone, the viscous fluid is broken (the viscosity is reduced) and recovered (returned to the surface). The gravel pack produced functions as a filter to separate formation sand and fines from produced fluids while permitting the produced fluids to flow into and through the well bore.
Another example of a well completion operation involves the use of viscous fluids known as drill-in fluids. The drill-in fluids are utilized when open-hole well bores are drilled into producing subterranean zones to minimize damage to the permeability of the zones and their ability to produce hydrocarbons. The drill-in fluid deposits filter cake on the walls of the well bore within the subterranean zone which prevents the drill-in fluid from being lost into the subterranean zone and prevents solids from entering the porosities of the subterranean zone. The filter cake which is made up of a bridging agent and gelled drill-in fluid is subsequently broken by contact with a clean-up fluid containing a delayed viscosity breaker.
After viscous treating fluids have been utilized in gravel packing treatments, drill-in treatments and other treatments conducted in subterranean zones penetrated by well bores, the viscous fluids utilized are broken, i.e., the viscous fluids are caused by delayed viscosity breakers therein to revert to thin fluids whereby they can readily be recovered from the subterranean zones.
While a variety of delayed viscosity breakers have been utilized heretofore, such breakers have often been hazardous to the health of personnel, are flammable and react with materials and components in the well bore which reduces the ability of the breaker to break viscous fluids. Also, a number of the heretofore used breakers have not been soluble in water which makes them difficult to utilize. Thus, there are continuing needs for improved delayed viscosity breakers, viscous fluids containing the breakers and methods of treating subterranean zones with viscous fluids containing improved delayed viscosity breakers.